This application is based on Japanese Patent Applications No. 2001-397640 filed on Dec. 27, 2001 and No. 2001-397823 filed on Dec. 27, 2001 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an air-fuel ratio control system for a multi-cylinder engine, more specifically, relates to a system for adjusting an air-fuel ratio of each cylinder by operation of intake control devices disposed on intake passages of cylinders respectively.
2. Description of Related Art
JP-A-9-133040 discloses an engine control system for controlling a temperature of a catalyst for purifying exhaust gas. The system operates at least one of cylinder in the engine under a relatively lean air-fuel ratio, and simultaneously operates the other one of cylinder in the engine under a relatively rich air-fuel ratio. The system performs the afore-mentioned control for speeding a warming up of the catalyst just after the engine has started. The system provides the different air-fuel ratios by adjusting amounts of fuel supply for each cylinder. However, the system still supplies the same amount of air into the cylinders.
JP-A-8-61052 discloses an engine control system for controlling a temperature of a catalyst for purifying exhaust gas too. The system operates at least one of cylinder in the engine under a relatively lean air-fuel ratio, and simultaneously operates the other one of cylinder in the engine under a relatively rich air-fuel ratio too. The system performs the afore-mentioned control at a certain intervals. As a result, the temperature of the catalyst is intermittently and temporally increased and the catalyst is refreshed due to the increased temperature. The system provides the different air-fuel ratios by adjusting amounts of fuel supply for each cylinder. However, the system still supplies the same amount of air into the cylinders.
In case of utilizing a NOx adsorb catalyst, it is needed to deoxidize adsorbed NOx by adjusting the air-fuel ratio rich temporally. For example, the air-fuel ratio for all cylinders is controlled as shown in FIG. 7. The Nox adsorb catalyst is advantageous for an in-cylinder direct injection engine or a lean-burn controlled engine which enables a relatively lean combustion and causes an increase of NOx instead of a decrease of HC and CO as shown in FIG. 8. However, such a NOx reducing method results in a fluctuation of engine output torque in accordance with the air-fuel ratio.
JP-A-4-41951 discloses an engine control system for operating one of cylinder groups under a lean air-fuel ratio. The system discloses a method for providing a lean air-fuel ratio by regulating a fuel amount and a method for providing a lean air-fuel ratio by regulating an air amount. However, the air amount is regulated by a throttle valve commonly disposed for a plurality of cylinders belonging to the cylinder group. Therefore, the plurality of cylinders is operated under the same air amount. In addition, each cylinder group has individual catalyst respectively. Therefore, the one catalyst is supplied with exhaust gas generated by combustions under the same air-fuel ratio. For this reason, the system is not capable of supplying both components originated lean air-fuel ratio and rich air-fuel ratio to the catalyst. Therefore, the system needs to employ additional controls in order reduce emissions.
It is an object of the present invention to provide an air-fuel ratio control system for a multi-cylinder engine being capable of reducing emissions.
It is another object of the present invention to provide an air-fuel ratio control system for a multi-cylinder engine being capable of speeding the warming up of the catalyst by adjusting an intake air amount for each cylinder.
It is another object of the present invention to provide an air-fuel ratio control system for a multi-cylinder engine being capable of reducing NOx by adjusting an intake air amount for each cylinder.
According to a first aspect of the present invention, an air-fuel ratio control system comprises a first intake control device for controlling an amount of intake air introduced into a first cylinder, and a second intake control device for controlling an amount of intake air introduced into a second cylinder. The system further comprises a controller which controls the first and second intake control devices to operate the first cylinder under a fuel lean air-fuel ratio and to simultaneously operate the second cylinder under a fuel rich air-fuel ratio, when the engine is operated under a predetermined condition. Therefore, it is possible to supply different exhaust gas contents from the first cylinder and the second cylinder. The first cylinder operated under the lean air-fuel ratio emits NOx or O2 more than the second cylinder. On the other hand, the second cylinder operated under the rich air-fuel ratio emits HC and CO more than the first cylinder. Therefore, it is possible to deoxidize NOx by HC and CO. Otherwise, it is possible to make HC and CO to react with O2 and to generate heat.